Organic electroluminescent devices (OLEDs) comprising organic semiconductors as functional emission materials are well known. An organic electroluminescent device is generally comprised of a pair of electrodes forming an anode and a cathode, and one layer or multiple layers comprising a hole transporting layer, emission layer (with an emissive material) and electron transporting layer. A voltage differential applied across the cathode and anode inject holes and electrons into the organic layer(s), which results in the formation of excitons within the emission material. The emission material then emits light when the excitons transition to the ground state.
New and improved phosphorescent materials for this type of application are continually being sought, since the luminous efficiency of OLEDs may potentially be improved by up to a factor of four when phosphorescent emitters are used in place of fluorescent emitters.
Organometallic complexes are increasingly being used as phosphorescent (triplet) emission materials in OLEDs (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). Organometallic complexes can possess strong intersystem crossing from the singlet to the triplet state and is partly responsible for their success, for application in OLEDs. Metal complexes can also used in other functions in OLEDs, such as matric materials, electron-transport materials, hole-blocking materials or as dopants for use in one or more layers of such devices.
Improvements are still required in the physical properties of phosphorescent OLEDs, for example in respect of the stability of the metal complexes, efficiency, operating voltage and lifetime for use of triplet emitters in high-quality and long-lived electroluminescent devices. Improvements are sought for red-, green- and blue-phosphorescent metal complexes used in OLEDs, and particularly for blue-phosphorescent metal complexes. In working towards this, the emission wavelength (λem), lifetimes (τ) and quantum yields (φP) are important considerations when searching for suitable phosphorescent materials.
Consequently, there is a need to identify compounds that can act as ligands for facilitating stability in phosphorescent metal complexes. There is also a need to identify novel metal complexes that can be used in OLEDs as emission materials or other functional materials such as matrix materials, hole-blocking materials and electron-transport materials.